Document sqlite -> mysql/postgresql migration and nltk_data environment variable in wsgi

I found it useful to pass the NLTK_DATA environment variable when hosting Bookie in wsgi, otherwise I would get a MissingCorpusException. Other than the NLTK_DATA envvar, the library seems to look for the nltk data only in a handful of hardcoded paths.
cool had never heard of taps before and this is kind of a cool use.